Semiconductor lasers are used in various portions of the transmitter, amplifier, and receiver chains of optical telecommunications systems. Typically, these transmitter and receiver semiconductor lasers utilize a frequency modulation (FM) scheme and coherent system architecture, and must have a relatively narrow linewidth (of around 0.007 pm, for example). The linewidth is the width of the laser frequency, and is much narrower than that of visible light, for example. The receiver semiconductor lasers are also referred to as “Local Oscillators” and are used to generate signals that are mixed with signals of interest in order to generate different frequencies. These amplifier semiconductor lasers utilize frequency stabilization and an optical feedback scheme using a fiber Bragg (FB) grating, for example. The amplifier semiconductor lasers are also referred to as “pump lasers” and are used to transfer energy into the gain medium of another laser, the pump power exceeding the lasing threshold of that laser. The transmitter and receiver semiconductor lasers are specially designed with large cavity structures in order to reduce their linewidth, or utilize a master/slave laser scheme. These components and concepts are well known to those of ordinary skill in the art.
As is also well known to those of ordinary skill in the art, an FB grating is a diffraction grating segment of an optical fiber that filters out particular wavelengths of light. This filtering is achieved by altering portions of the optical fiber core such that their indices of refraction are slightly higher than normal. This is accomplished using an intense ultraviolet (UV) source and a photomask, for example, combined with various receptive optical fiber core material compositions. As a result, the FB grating transmits most wavelengths of light, but reflects the particular wavelengths of light. The FB grating typically has a sinusoidal index of refraction variation over a defined length and may include one or more “chirps.”
The schemes and architectures described above are less than desirable due to the fact that the specially designed cavity structure and the like are relatively costly and there is no real-time mechanism to control and adjust this laser parameter. In addition, for the pump lasers, the optical feedback is relatively unstable due to the location of the mirror (FB grating). This mirror is also susceptible to temperature and environmental changes. Thus, what is needed is a semiconductor laser utilizing an effective real-time linewidth reduction method. In addition, this semiconductor laser should be compact in nature.